Aerosol dispensers have been available for a number of years for application of hair sprays and other personal care products, as well as for paints, lubricants, insecticides and a multitude of other liquid products.
In the earlier days of aerosol marketing, "Freon" blends were widely used as propellants, principally because they were not flammable and were relatively nontoxic. However, in recent years, it was discovered that "Freons" when released into the atmosphere, migrated to the upper stratosphere and contributed to the depletion of ozone. Since ozone shields the surface of the earth against penetration of solar ultraviolet radiation, the use of "Freon" propellants was believed by many to have contributed to climate changes, as well as to an increased incidence of skin cancers and cataracts. Accordingly, the U.S. Environmental Protection Agency (E.P.A.) banned the further use of "Freon" propellants. This forced aerosol packagers to select from three alternative propellant systems, all of which had serious disadvantages. First, were various hydrocarbons, consisting of blends of propane and butane mixtures. However such hydrocarbon propellants are extremely flammable, and are in the chemical class known as volatile organic compounds, which are recognized as a significant cause of lower atmospheric air pollution or smog. Therefore, in even moderately populated areas of the country severe restrictions have been placed on the use of such compounds in many operations. While, because of their size, aerosol packages have generally been exempt so far from such restrictions, there is a growing indication that, at least in a number of highly populated problem areas, their eventual phaseout is inevitable.
A second alternative involves the use of gaseous carbon dioxide as the propellant. Being non-flammable and nontoxic, carbon dioxide gas is an excellent medium for this purpose, but such systems have limited utility because of the relatively small quantity of the pressurized gas that can be accommodated in the head space of the aerosol container. Moreover, limitations of pressure to relatively low maximum allowable safe levels greatly restrict the energy content of the propellant gas, making it suitable for use with just very low viscosity products. Also, as the liquid product and propellant gas are evacuated, the delivery pressure diminishes further, usually resulting in a change in spray pattern.
The third alternative system presently available involves the use of hydrochlorofluorocarbons (HCFC's) and Dimethylether. However, such propellants are extremely expensive and, at least in the case of dimethylether, flammable. Moreover, it is suspected, that such propellants may have some ozone depleting effect.
In addition to the selection of a gas propellant, further problem in the aerosol packaging industry resides in the disposing of the used aerosol containers, particularly for large scale commercial users. After the product has been used up, the container remains charged with the propellant, and where the propellant is a flammable gas, the container is considered hazardous waste, even where the product is a water-based liquid.
Also, in many present systems there is the problem of co-mixing the product and the propellant in a common container. In such case, problems of incompatibility are often encountered, particularly with water-based products.